The invention relates to a high voltage DC breaker apparatus configured to break a fault current occurring in a high voltage DC conductor, said apparatus comprising                a current limiting arrangement having at least one section with at least one semiconductor device of turn-off type and at least one arrester connected in parallel therewith, said at least one section being configured to be connected in series with said DC conductor,        means configured to detect occurrence of a fault current in said DC conductor, and        a unit configured to control breaking of said fault current upon detection of occurrence thereof, said control including turning off all semiconductor devices of said arrangement.        
High voltage means a voltage ≧10 kV and often a voltage of several hundreds kV with respect to ground.
Such a high voltage DC breaker apparatus may be arranged in a high DC voltage carrying system for obtaining breaking of fault currents occurring in a said DC conductor, in general as a consequence of short circuits to earth. It is then of great importance to be able to at a very short notice, such as in the order of a few hundreds μs, upon occurrence of such a fault current limit this fault current for preventing severe impacts upon equipment connected to the system, which is the reason for using semiconductor devices of turn-off type, which may open within a few μs, as switches in a current limiting arrangement of such a breaker apparatus.
An apparatus of this type is known through U.S. Pat. No. 5,999,388 and a similar known such apparatus is shown in appended FIG. 1, in which the current limiting arrangement 1 has eight sections 2 connected in series, and each section has at least one semiconductor device of turn-off type 3, here an IGBT (Insulated Gate Bipolar Transistor) and at least one arrester 4 connected in parallel therewith. Each IGBT symbol may in fact stand for a large number, such as 10, of IGBT:s connected in series, and the same may apply to the arrester 4. A rectifying member in the form of at least one diode will also be connected in antiparallel with each semiconductor device shown in the figures of this disclosure. However, these diodes have for simplicity reasons been omitted in the figures. The arrangement 1 is configured to be connected in series with a DC conductor 5 on high voltage potential. It is shown how the apparatus has means 6 for detecting occurrence of a fault current in the DC conductor and sending information thereabout to a control unit 7 configured to control breaking of said fault current by controlling the semiconductor devices 3.
Turning off the IGBT:s 3 of an individual section 2 during a fault inserts the corresponding arrester 4 into the line. The voltage across the section is given, almost independent of the fault current, by the protective level of that arrester or arrester bank (plurality of arresters) of said section. Assuming that the protective voltage level of the complete arrangement shown in FIG. 1 corresponds to 1.60 times the DC voltage intended for the DC conductor, insertion of more than five arresters (arrester banks) will reduce the fault current. Insertion of five arrester banks will limit the fault current to the current level at the breaking time instance.
Although such solid state DC breakers, i.e. based on semiconductor switches, respond almost instantaneously to the demand of the control and protection system, a disadvantage of such a breaker apparatus is that it has a large number of semiconductor devices, such as IGBT:s, to be connected in series for being able to take a voltage corresponding to the protective voltage of the arrester banks, which results in considerable costs and also power transfer losses when the current of the DC conductor flows through the semiconductor devices.
The losses of mechanical DC breakers, based on standard AC breakers as found in High Voltage Direct Current applications, are in comparison to this very small. The disadvantage of such existing mechanical DC breaker solutions is, however, the time delay between the control signal from the protection system and the breaking action. Even with a modified standard AC breaker it will take 10-20 ms until the mechanical contacts are opened, which is too slow for application in a high voltage DC conductor, such as in a DC grid. This is the reason why solid state DC breakers with the appearance shown in FIG. 1 have been used so far for realizing high voltage DC breaker apparatuses.